4.2.1 Project Summary and Relevance This proposal seeks support to develop a fundamentally new approach to amide and peptide chemical synthesis, one that complements existing methods based on dehydrative amide synthesis using carboxylic acids and amines. Bromonitroalkanes serve as carboxylic acid surrogates in a direct amide synthesis that utilizes an amine acceptor and an activating agent (a halonium ion). The concise preparation of amides derived from nonnatural amino acids, common constituents of biologically active linear and cyclic peptides that have been isolated from natural sources, is a central theme. Without this new paradigm, alternative chemical methods would provide access to the desired amides at rising cost due to the large number of steps required to prepare complex peptides, and the contamination of intermediates and products by stereoisomers that are difficult to remove. The practical chemical synthesis of biologically active peptides is an immediate goal. In the short term, peptides of modest size (~10 residues) will be prepared and diversified. In the long term, this innovative approach to amide synthesis will be used in combination with conventional methods to provide access to large peptides (e.g. biologics) modified site-specifically with nonnatural amino acids.